This application claims the benefit of Japanese Application No. 2001-281374 filed Sep. 17, 2001.
The present invention relates to a Doppler signal processing method and apparatus and an ultrasonic diagnostic apparatus, and more particularly to a Doppler signal processing method and apparatus and an ultrasonic diagnostic apparatus for cutting time-domain Doppler signals of a predetermined length, window-processing the cut signals, transforming the window-processed signals into frequency-domain signals by Fourier transformation, inversely transforming the frequency-domain signals into a time-domain signal by inverse Fourier transformation after performing appropriate processing such as filtering on the signals in the frequency domain, and outputting the time-domain signal as an acoustic signal.
An ultrasonic imaging apparatus scans the interior of a subject with an ultrasonic beam; receives echoes; acquires image data corresponding to the intensity of the echoes; and thereby produces a so-called B-mode image. This process is sometimes referred to as B-mode imaging.
Moreover, the ultrasonic imaging apparatus acquires a Doppler signal of the echoes, and produces color images representing the dynamics of blood flow etc., i.e., so-called color Doppler images, based on the Doppler signals. The color Doppler images produced include a color flow mapping image that represents a two-dimensional distribution of the velocity of blood flow etc., and a power Doppler image that represents a two-dimensional distribution of the power of the Doppler signal. This process is sometimes referred to as color Doppler imaging.
The ultrasonic diagnostic apparatus also performs Fourier transformation on the Doppler signal of echoes from a sample volume defined in the interior of the subject to obtain a frequency spectrum. This process is sometimes referred to as point Doppler measurement.
The results of the B-mode imaging, color Doppler imaging and point Doppler measurement are displayed as respective images for use as visual information to diagnose the subject. The point Doppler measurement also outputs the Doppler signal as an acoustic signal via an audio device such as a speaker, for use as acoustic information for the diagnosis.
The signal input to the audio device is a time-domain Doppler signal before the Fourier transformation or a signal obtained by inversely Fourier-transforming the frequency spectrum back into a time-domain signal.
An FFT (fast Fourier transformer) is used for the Fourier transformation from the time domain into the frequency domain. An iFFT (inverse fast Fourier transformer) is used for the inverse Fourier transformation from the frequency domain back into the time domain.
In performing the Fourier transformation by the FFT, a temporally continuous Doppler signal is cut into signals of a predetermined length, and the cut signals are subject to window-processing for mitigating discontinuity between the signals at the cut boundaries. The window employed is, for example, a Hanning window. The window-processed Doppler signal is amplitude-modulated according to the profile of the window.
Accordingly, the signal obtained from the inverse Fourier transformation becomes a Doppler signal amplitude-modulated according to the window profile, and the acoustic signal based on such a Doppler signal is inaccurate.
Therefore, an object of the present invention is to provide a Doppler signal processing method and apparatus and an ultrasonic diagnostic apparatus for window-processing the time-domain Doppler signals, translating the signals to a frequency domain, and then translating them back to the time domain, the Doppler signal processing method and apparatus and the ultrasonic diagnostic apparatus being capable of removing an effect of the window processing.
(1) The present invention, in accordance with one aspect thereof for solving the problem, is a Doppler signal processing method for cutting signals of a predetermined length from a time-domain Doppler signal; performing window processing on said cut signals; transforming said window-processed signals into frequency-domain signals by Fourier transformation; inversely transforming said transformed frequency-domain signals into a time-domain signal by inverse Fourier transformation after performing predefined processing on said signals; and outputting said inversely transformed time-domain signal as an acoustic signal, characterized in that: said window processing is performed using a window having a profile with a flat top; and said cutting from the Doppler signal is performed such that a former cut signal and a latter cut signal overlap in a portion corresponding to the flat portion of said window.
(2) The present invention, in accordance with another aspect thereof for solving the problem, is a Doppler signal processing apparatus for cutting signals of a predetermined length from a time-domain Doppler signal; performing window processing on said cut signals; transforming said window-processed signals into frequency-domain signals by Fourier transformation; inversely transforming said transformed frequency-domain signals into a time-domain signal by inverse Fourier transformation after performing predefined processing on said signals; and outputting said inversely transformed time-domain signal as an acoustic signal, characterized in comprising: window processing means for performing said window processing using a window having a profile with a flat top; and signal cutting means for performing said cutting from the Doppler signal such that a former cut signal and a latter cut signal overlap in a portion corresponding to the flat portion of said window.
(3) The present invention, in accordance with still another aspect thereof for solving the problem, is an ultrasonic diagnostic apparatus for transmitting ultrasound and acquiring a Doppler signal of an echo of the ultrasound; cutting signals of a predetermined length from said Doppler signal in a time domain; performing window processing on said cut signals; transforming said window-processed signals into frequency-domain signals by Fourier transformation; inversely transforming said transformed frequency-domain signals into a time-domain signal by inverse Fourier transformation after performing predefined processing on said signals; and outputting said inversely transformed time-domain signal as an acoustic signal, characterized in comprising: window processing means for performing said window processing using a window having a profile with a flat top; and signal cutting means for performing said cutting from the Doppler signal such that a former cut signal and a latter cut signal overlap in a portion corresponding to the flat portion of said window.
In the invention of the aspects (1)-(3), since the window processing is performed using a window having a profile with a flat top and the cutting from the Doppler signal is performed such that a former cut signal and a latter cut signal overlap in a portion corresponding to the flat portion of the window, the Doppler signals translated from the frequency domain to the time domain connect at the flat portion of the window. Thus, an effect of the window processing can be removed.
Preferably, the profile of the window is generally trapezoidal so that discontinuity between the signals due to the cutting can be mitigated.
(4) The present invention, in accordance with still another aspect thereof for solving the problem, is a Doppler signal processing method for cutting signals of a predetermined length from a time-domain Doppler signal; performing window processing on said cut signals; transforming said window-processed signals into frequency-domain signals by Fourier transformation; inversely transforming said transformed frequency-domain signals into a time-domain signal by inverse Fourier transformation after performing predefined processing on said signals; and outputting said inversely transformed time-domain signal as an acoustic signal, characterized in comprising: performing amplitude modulation with a property opposite to that of amplitude modulation by said window processing before outputting said inversely-transformed time-domain signal as the acoustic signal.
(5) The present invention, in accordance with still another aspect thereof for solving the problem, is a Doppler signal processing apparatus for cutting signals of a predetermined length from a time-domain Doppler signal; performing window processing on said cut signals; transforming said window-processed signals into frequency-domain signals by Fourier transformation; inversely transforming said transformed frequency-domain signals into a time-domain signal by inverse Fourier transformation after performing predefined processing on said signals; and outputting said inversely transformed time-domain signal as an acoustic signal, characterized in comprising: amplitude modulating means for performing amplitude modulation with a property opposite to that of amplitude modulation by said window processing before outputting said inversely-transformed time-domain signal as the acoustic signal.
(6) The present invention, in accordance with still another aspect thereof for solving the problem, is an ultrasonic diagnostic apparatus for transmitting ultrasound and acquiring a Doppler signal of an echo of the ultrasound; cutting signals of a predetermined length from said Doppler signal in a time domain; performing window processing on said cut signals; transforming said window-processed signals into frequency-domain signals by Fourier transformation; inversely transforming said transformed frequency-domain signals into a time-domain signal by inverse Fourier transformation after performing predefined processing on said signals; and outputting said inversely transformed time-domain signal as an acoustic signal, characterized in comprising: amplitude modulating means for performing amplitude modulation with a property opposite to that of amplitude modulation by said window processing before outputting said inversely-transformed time-domain signal as the acoustic signal.
In the invention of the aspects (4)-(6), since amplitude modulation with a property opposite to that of amplitude modulation by the window processing is performed before outputting the time-domain signal as the acoustic signal obtained by the inverse transformation, an effect of the window processing can be removed.
The window for performing the window processing on said cut signals is preferably a Hanning window so that discontinuity of the signals at the cut boundaries can be eliminated.
The predefined processing is preferably filtering processing so that unwanted signal components can be removed.
The predefined processing is preferably zero-shifting processing so that aliasing of the Doppler signal can be corrected.
The predefined processing is preferably correction processing of the output characteristics of said acoustic signal so that fidelity of the acoustic signal can be improved.
Therefore, the present invention can provide a Doppler signal processing method and apparatus and an ultrasonic diagnostic apparatus for window-processing time-domain Doppler signals, translating the signals to a frequency domain, and then translating them back to the time domain, the Doppler signal processing method and apparatus and the ultrasonic diagnostic apparatus being capable of removing an effect of the window processing.
Further objects and advantages of the present invention will be apparent from the following description of the preferred embodiments of the invention as illustrated in the accompanying drawings.